In the broadband access network with a larger and larger scale, most of existing Local Area Networks (LANs) operate on a network of 100 Mbit/s, and many large-scale commercial companies are transiting to Gigabit Ethernet (GE). However, on the metropolitan area core network and the metropolitan area edge network, Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH)/Gigabit Ethernet (GE) each have sufficient bandwidth capacity, which results in a serious bandwidth bottle at the access network part. Compared with cable transmission, optical fiber transmission has advantages including large capacity, small loss and strong Anti-electro-magnetic interference capability. Therefore, with the decreased cost of optical fiber transmission, the utilization of optical fiber is an inevitable trend. The access network segment representing “the last mile” part has requirements such as very low cost, simple structure and convenient implementation, which challenges the technical implementation. Passive devices are adopted in the Passive Optical Network (PON), which is the most potential technique to implement the broadband optical access network.
By the content that is born, the PON technique is classified into ATM Based PONs (APON), Ethernet Based PONs (EPON) and Gigabit PONs (GPON).
The broadband access technique for the Passive Optical Network (PON) has the following advantages:
In the passive optical network, there is no active device between the central office and the customer premise network. Alternatively, passive optical devices are inserted in the network, and transmitted flows are led by separating the power of optical wavelength throughout the path. With this alternation, service providers do not need to supply power to active devices and maintain the active devices in the transmission loop, which saves cost for the service providers greatly. The passive optical splitter and coupler only serve to transfer and restrict the light without the need of power supply and information processing, and have an unlimited Mean Time Between Failure (MTBF) which can reduce the maintenance cost for the service providers completely.
As illustrated in FIG. 1, the passive optical network usually includes an Optical Line Terminal (OLT) located in the Central Office (CO) and a series of Optical Network Units (ONUs) located in the customer premise. Between these devices, there is an Optical Distribution Network (ODN) consisting of optical fibers, and passive optical splitters or couplers. In a PON, a single optical fiber may be led from a serving switching office to a bandwidth service sub-area or an office area, and then the main optical fiber is split into several branches to respective buildings or service devices by using a passive optical splitter or coupler. In this manner, multiple users can share the relative expensive optical fiber link which is from the switching office to the customer premise, and the cost of Fiber To The Building (FTTB) and Fiber To The Home (FTTH) is therefore reduced greatly.
With the techniques of APON, Broadband Passive Optical Network (BPON), EPON or GPON to be standardized, the speed of 155 Mbit/s, 622 Mbit/s, 1.25 Gbit/s, or 2.25 Gbit/s can be provided on backbone optical fibers of the PON. To support voice, data and video applications concurrently, the bandwidth for each user may be configured statically, or the bandwidth for each user may be configured dynamically.
The principle of data transmission in the PON is as follows:
In accordance with the PON technique, Time Division Multiple Access (TDMA) technique is used in the upstream link path, and the office-end device OLT performs time window authorization for the respective ONUs in the network, i.e. authorizes different ONUs to send upstream data during different time periods, so that the upstream data from the different ONUs may not conflict. Therefore, the optical module device in each ONU terminal is controlled by PON protocol, to open and close the sending function of the optical module intermittently (i.e. so-called “burst” sending). The receiving function of the optical module of the ONU terminal keeps in an open state, so that the ONU can receive time window authorization information from the OLT.
Usually, message frames of time window authorization sent from the OLT to each ONU each include two kinds of information: starting time at which the ONU is allowed to open the optical module for sending, and duration time in which the ONU opens the optical module for sending. The OLT allocates an authorized time window for each ONU dynamically in accordance with bandwidth requirements of each ONU, so that use bandwidth of each ONU can change dynamically and complies with a preset bandwidth policy. The calculation and scheduling process is called Dynamic Bandwidth Allocation (DBA) algorithm. The OLT can set a mode in which an ONU obtains an authorized time window through the DBA algorithm, e.g. periodical cycle mode. When an ONU registers with the PON, the OLT performs time synchronization with the ONU, so that the two parties recognize a common time.
The ONU supports multiple services and some of the services are activated in a low-speed connection state for a long time with a small bandwidth occupied (e.g. the Keep Alive mechanism of terminals and the office end in VoIP service). At present, ONU terminals are kept in an operating state for a long time, energy consumed during the operation is fixed, and power consumption can not be adjusted automatically. The energy is wasted greatly when services requiring low power consumption are activated for a long time.
The energy consumption includes three aspects: office-end device, terminal devices, and attenuation consumption of in-between ODN. Consumption of office end and terminals is large, which is caused by operation of devices. Consumption of the in-between ODN is small, which is caused by attenuation of laser in optical fibers.
Because the PON system is proposed for a short time and actual PON devices are not applied to operate, there is no relevant energy saving technique of PON terminals at present.